Time-resolved spectroscopic measurements were made of the decay of a laser-produced discharge in hydrogen over a range of pressures from one to 70 atmospheres. The plasma temperature, which ranged from about 10 ⁵ to 10^{4}\deg K during the Q -switched laser pulse to several microseconds afterward, was obtained from the ratio of the intensity of H α to that of nearby continuum radiation. The electron density, which ranged from about 10 ¹⁹ to 10 ¹⁷ per cm ³ was obtained from the time history of the Stark linewidths of H α Since the high collision rates ensure approximate local thermodynamic equilibrium, several important characteristics of the plasma could be calculated from the measured temperature and density, such as optical absorption at the laser frequency due to inverse bremsstrahlung and photoionization. The energy deposited by the laser was measured from the decrease of the transmitted laser beam. Scattered radiation seemed small. The observed laser absorption is about two orders of magnitude larger than that calculated for a Maxwellian electron distribution. The calculated line absorption for H α was consistent with self-absorption observed in the H α spectral profile. The ratio of the measured watts of H α to the calculated watts/cm ³ yielded values for the volume of the luminous region which generally decreased with gas pressure from 10 ^-3 to 10 ^-4 cm ³ , as observed.

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